The invention relates generally to gas turbine engines, and, more specifically, to film cooling therein. In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. Energy is extracted from the gases in a high pressure turbine (HPT), which powers the compressor, and in a low pressure turbine (LPT), which powers a fan in a turbofan aircraft engine application, or powers an external shaft for marine and industrial applications.
Engine efficiency increases with temperature of combustion gases. However, the combustion gases heat the various components along their flowpath, which in turn requires cooling thereof to achieve a long engine lifetime. Typically, the hot gas path components are cooled by bleeding air from the compressor. This cooling process reduces engine efficiency, as the bled air is not used in the combustion process.
Gas turbine engine cooling art is mature and includes numerous patents for various aspects of cooling circuits and features in the various hot gas path components. For example, the combustor includes radially outer and inner liners, which require cooling during operation. Turbine nozzles include hollow vanes supported between outer and inner bands, which also require cooling. Turbine rotor blades are hollow and typically include cooling circuits therein, with the blades being surrounded by turbine shrouds, which also require cooling. The hot combustion gases are discharged through an exhaust which may also be lined, and suitably cooled.
In all of these exemplary gas turbine engine components, thin metal walls of high strength superalloy metals are typically used for enhanced durability while minimizing the need for cooling thereof. Various cooling circuits and features are tailored for these individual components in their corresponding environments in the engine. In addition, all of these components typically include common rows of film cooling holes.
A typical film cooling hole is a cylindrical bore inclined at a shallow angle through the heated wall for discharging a film of cooling air along the external surface of the wall to provide thermal insulation against the hot combustion gases which flow thereover during operation. The film is discharged at a shallow angle over the wall outer surface to minimize the likelihood of undesirable blow-off thereof, which would lead to flow separation and a loss of the film cooling effectiveness.
Film performance is dictated by effective coverage. If the diffuser is too aggressive in expansion, it results in the flow stalling and a degree of jetting at the outlet of the diffuser. This jetting is detrimental as it reduces the effective coverage of the film and introduces secondary mixing that degrades the film. The side diffusion angle and length govern the physical coverage of the diffuser at the outlet. The layback diffusion angle governs how the film transitions from the covered diffusion portion to the mainstream flow.